While looking up Pentecost music, I have found that some Hymn Books, i.e. Celebration, Iubilate Deo, omit the last verse of the Veni Creator
while others, such as The Westminster Hymnal and Plainsong for Schools, include it. Bizarrely, Celebration Hymnal does include this verse in
the standard translation-- Come Holy Ghost. The verse begins 'Deo Patri sit gloria' in Latin 'All glory to the Father be', in English.
Can anyone explain this anomaly?

Seems bizarre to omit the last verse given the symbolism of having 7 verses in this hymn. Pentecost is one of the few times where a 7 verse hymn is sung in full in our parish; not sure what the people make of it though!

- and was it not the norm (if not actually a "rule") that if a hymn which concluded with a doxology needed to be shortened for practical reasons the doxology was never omitted? (Groping in distant memories here!)

Looks like the hymn originally only had six verses. It's an oddity of the seven-verse version that the final two verses are both trinitarian in character. Raniero Cantalamessa says (in Come, Creator Spirit: Meditations on the Veni Creator):

There are some who have tried to show that there is a parallel between each of the seven gifts and the seven verses of the Veni Creator, but it is a well-known fact that the hymn had originally six verses: the seventh (Deo Patri sit Gloria...) is a doxology from the repertoire added later.